1. Field of the Invention
This invention relates in general to motors, and more particularly to methods for containing oil and/or grease loss in ball bearings of spindle motors.
2. Description of Related Art
Disk drives are computer mass storage devices from which data may be read and/or to which such data may be written. In general, they comprise one or more randomly accessible rotating storage media, or disks, on which data is encoded by various means. In magnetic disk drives, data is encoded as bits of information including magnetic field reversals grouped in tracks on the magnetically-hard surface of the rotating disks. The disks are stacked in a generally parallel and spaced-apart relationship and affixed at their inner diameter ("ID") to a common hub which is rotationally coupled to a stationary spindle shaft by a pair of bearings, typically ball bearings.
Spindle motors for direct access storage devices (DASDs) currently utilize high quality ball bearings that are lubricated for life using a metered amount of grease. The ball bearings are generally fitted with non-contact rubber shields one on each side. The shields have a small radial gap (0.2 to 0.25 mm) at the ID side, which is principally determined by the manufacturing tolerances and are pressed lightly into the groove in the outer ring. The elastomer in the shield is compressed in the process to provide a sealing effect. In these shields, metal (steel) backing is bonded to the elastomer. The metal backing side faces the inside of the bearing the conventional approach used in most bearings today. The elastomer widely used in ball bearings today is Nitrile Butadiene Rubber (NBR).
Recent experiments and experiences have shown significant oil loss and grease loss for the above spindle motors when tested under accelerated conditions of temperature and speed. It appears that three of the significant sources of this loss are: (1) from the shield outer diameter ("OD") contact area, (2) through the shield elastomeric material itself, and (3) from the shield ID gap.
The plausible mechanism of loss through shield/outer ring contact area is by creep or migration by virtue of insufficient sealing effect. The mechanism of oil loss through the shield is due to the permeation of oil molecules through the elastomer matrix. The plausible mechanisms are loss through aerosolization and loss via surface creep or migration and subsequent appearance of oil droplets on the outside surface of the shield. These droplets could then be either slung out due to centrifugal forces and/or evaporate. Loss through aerosolization increases greatly for increased rotational speeds and for higher temperatures because of reduced viscosity. Thus the ability to achieve higher than 10000 rpm for ball bearing based spindle motors depends on adequate containment of the grease and base oil within the bearings.
Previous designs in this area include various shield designs of the non-contacting type. FIG. 1 shows the type of shield 100 currently used in most types of DASD spindle motors in use today. FIG. 1 shows a cross sectional view of the shield 100. The metal backing 100 is bonded to the elastomer shield 120. The metal backing 110 faces the inside of the bearing in the conventional approach used in most bearings today. The elastomer 120 widely used in ball bearings today is Nitrile Butadiene Rubber (NBR). A first end 130 of the shield 100 contacts grooves at the outer race (not shown). A second end 140 forms a gap 142 at the inner race 144.
Significant oil/grease loss can also occur at the shield ID gap in the case of high speed (10000 rpm ) ball bearings especially at the upper end of the temperature specification (70 to 80.degree. C.). Prior shield designs of the non-contacting type however exhibit less than desirable oil/grease containment characteristics. For example, FIGS. 2 and 3 show two types of shields 200, 300 currently being used in two different types of spindle motors. FIGS. 2 and 3 show a cross sectional view of the ball bearing, but because of symmetry only a portion of the view is shown.
In FIG. 2, a shield 210 faces the outer diameter 212 of the inner race 214. Also shown is the cage 220 and the ball bearing 222. In FIG. 3, the ID of the shield 310 faces a step 330 on the inner race 314. The ID gap 318 offers some resistance to aerosol loss based on the gap 318 and the thickness of the shield 310. The shield 310 in FIG. 3 offers a slightly higher resistance to flow when compared to the shied 210 of FIG. 2 by virtue of the small step 330 in the inner race 314. Whether or not a step 314 is possible is determined by thickness of the inner race 314.
Stainless steel shields are also used in ball bearings for spindle motors. Stainless steel shields offer some significant advantages over rubber shields: (1) Smaller gaps at the ID are possible with stainless steel shields because no molding process is involved. (2) Stainless steel shields can be much thinner than rubber shields (about one-half) because of higher strength, which would permit increased grease amounts to be charged into the bearings. In addition, stainless steel shields eliminate the mechanism of foil loss and of course a major source of outgassing (NBR) is eliminated with the use of stainless steel shield.
FIG. 4 shows the front view of a commercially available stainless steel shield 400 for a 5.times.13 ball bearing, e.g. a stainless steel shield as manufactured by NSK Corporation, Japan. Eight equally spaced slits or slots 410 approximately 0.12 mm wide are made in the shield 400 for ease of insertion in the bearing outer ring groove. The stainless steel shields for a larger 6.times.15 bearing is of similar design also.
FIG. 5 shows the rear view 500 of the same shield. Protrusions 520 measuring approximately 0.08 mm are clearly visible at the positions corresponding to the slots 410 illustrated in FIG. 4. Detailed and closer observations of these protrusions 520 reveal that the material 522 immediately next to them is not affected in terms of profile. Upon insertion of such a shield contact occurs at the protrusions 520 and far away from the protrusions. But, in the in-between areas 522 sufficient metal to metal contact is not likely to occur. This poses a major source of oil leakage path.
It can be seen that there is a need for improved shield designs for ball bearing based spindle motors that provide improved containment of grease and the base oil within the bearings at accelerated conditions of temperature and speed.